The present invention relates to a method of producing metal composite materials such as cemented carbide.
Cemented carbide and titanium-based, carbonitride alloys often referred to as cermets consist of hard constituents based on carbides, nitrides and/or carbonitrides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W in a binder phase essentially based on Co and/or Ni. They are made by powder metallurgical methods of milling a powder mixture containing powders forming the hard constituents and binder phase, pressing and sintering.
The milling operation is an intensive milling in mills of different sizes and with the aid of milling bodies. The milling time is on the order of several hours to several days. Such procedures are believed to be necessary in order to obtain a uniform distribution of the binder phase in the milled mixture. It is further believed that the intensive milling creates a reactivity of the mixture which further promotes the formation of a dense structure.
British Patent No. 346,473 discloses a method of making cemented carbide bodies. Instead of milling, the hard constituent grains are coated with binder phase via an electrolytic method, pressed and sintered to a dense structure. This and other similar methods are, however, not suited for cemented carbide production on a large industrial scale, and milling is almost exclusively used within the cemented carbide industry today. However, milling has its disadvantages. For instance, during the long milling time wear of the milling bodies contaminates the milled mixture and it is necessary to compensate for such contamination. The milling bodies can also break during milling and remain in the structure of the sintered bodies. Furthermore, even after an extended milling a random rather than an ideal homogeneous mixture may be obtained. Moreover, in order to ensure an even distribution of the binder phase in the sintered structure sintering has to be performed at a higher temperature than would otherwise be necessary.
The properties of the sintered metal composite materials containing two or more components depend to a great extent on how well the starting materials are mixed. An ideal mixture of particles of two or more kinds especially if one of the components occurs as a minor constituent (which is the case for the binder phase in ordinary metal composite materials) is difficult to obtain. In practice, after extended mixing a random rather than an ideal homogeneous mixture is obtained. In order to obtain an ordered mixing of the components in the latter case, the minor component can be introduced as a coating. The coating can be achieved by the use of various chemical techniques. In general, it is required that some type of interaction between the coated component and the coating be present, i.e., adsorption, chemisorption, surface tension or any type of adhesion.
U.S. Pat. No. 4,539,041 discloses the well known polyol process. This process is being used today for the manufacture of cobalt and nickel metal powders with a small particle size. These metal powders can, for example, be used for the production of hard materials as disclosed in WO SE92/00234. In this process a number of transition metals such as Co, Ni, Cd, Pb as well as more easily reducible metals such as Cu and precious metals can be reduced to the metallic state by a polyol such as: ethylene glycol, diethylene glycol or propylene glycol. A complete reduction is obtained after about 24 hours and the metal is precipitated as a fine powder. The reaction proceeds via dissolution with the polyol functioning both as a solvent and as a reducing agent at the same time.